Pad Design for Reliability Enhancement in Packages

ABSTRACT

A package includes a corner, a device die, a molding material molding the device die therein, and a plurality of bonding features. The plurality of bonding features includes a corner bonding feature at the corner, wherein the corner bonding feature is elongated. The plurality of bonding features further includes an additional bonding feature, which is non-elongated.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is continuation of U.S. patent application Ser. No.14/865,832, filed Sep. 25, 2015, and entitled “Pad Design forReliability Enhancement in Packages,” which is a continuation-in-partapplication of the following commonly-assigned U.S. patent application:application Ser. No. 14/613,997, filed Feb. 4, 2015, and entitled “PadDesign for Reliability Enhancement in Packages,” which further claimsthe benefit of the following U.S. Provisional Application No.62/011,432, filed Jun. 12, 2014, and entitled “Integrated CircuitPackage Pad and Method of Forming Same,” which applications are herebyincorporated herein by reference.

BACKGROUND

In the packaging of integrated circuits, there are various types ofpackaging methods and structures. For example, in a conventionalPackage-on-Package (POP) process, a top package is bonded to a bottompackage. The top package and the bottom package may also have devicedies packaged therein. By adopting the PoP process, the integrationlevel of the packages is increased.

In an existing PoP process, the bottom package, which includes a devicedie bonded to a package substrate, is formed first. A molding compoundis molded to the package substrate, wherein the device die is molded inthe molding compound. The package substrate further includes solderballs formed thereon, wherein the solder balls and the device die are ona same side of the package substrate. The solder balls are used forconnecting the top package to the bottom package.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of a package in accordancewith some embodiments;

FIG. 2 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein corner openings are elongated connectors;

FIG. 3 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein a plurality of elongated openings is distributed to each corner;

FIG. 4 illustrates the top view of a package and non-solder openings ofthe package in accordance with some embodiments, wherein elongatedopenings and non-elongated openings are distributed depending on theirdistances to a neutral-stress point of the package;

FIG. 5 illustrates a top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein inner openings adjacent to the corners of an underlying devicedie are elongated;

FIG. 6 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein inner openings adjacent to the corners of an underlying devicedie group are elongated;

FIG. 7 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein a plurality of inner openings adjacent to each corner of anunderlying device die is elongated;

FIG. 8 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein a plurality of inner openings adjacent to each corner of anunderlying device die group is elongated;

FIG. 9 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein openings in corner rectangular regions are elongated;

FIGS. 10A and 10B illustrate top views of some exemplary elongatedopenings;

FIGS. 11A and 11B illustrate top views of some exemplary non-elongatedopenings in accordance with some embodiments;

FIGS. 12A and 12B illustrate the exemplary shapes of elongated openingsand their respective underlying metal pads in accordance with someembodiments;

FIG. 13 illustrates the metal pads on a package that is bonded to thepackage with elongated openings;

FIGS. 14 through 18A illustrate the cross-sectional views ofintermediate stages in the formation of a package in accordance withsome embodiments;

FIGS. 18B and 18C illustrate the top views of a portion of a package inaccordance with some embodiments;

FIGS. 19A and 19B illustrate the top views of some exemplary elongatedbonding pads in accordance with some embodiments;

FIGS. 20A and 20B illustrate top views of some exemplary non-elongatedbonding pads in accordance with some embodiments;

FIGS. 21 through 25 illustrate the cross-sectional views of intermediatestages in the formation of a package in accordance with someembodiments;

FIG. 26 illustrates the top view of a portion of a package in accordancewith some embodiments; and

FIGS. 27A and 27B illustrate bottom views of some exemplary metal padsin a top package in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

An integrated fan-out package and the structure for improving thereliability of the package are provided in accordance with variousexemplary embodiments. The variations of the embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements.

FIG. 1 illustrates a cross-sectional view of package 40 in accordancewith some exemplary embodiments of the present disclosure. Package 400includes bottom package 100 and top package 200 over and bonded tobottom package 100. Both bottom package 100 and top package 200 arepre-formed, and are then bonded to each other to form a package onpackage structure. In accordance with some embodiments of the presentdisclosure, bottom package 100 includes device dies 102 (including 102Aand 102B), with the front sides of device dies 102 facing down andbonded to Redistribution Lines (RDLs) 112. In alternative embodiments,bottom package 100 includes a single device die or more than two devicedies. Device dies 102 may include semiconductor substrates 108, andintegrated circuit devices (such as active devices, which includetransistors, for example) 104 at the front surface (the surface facingdown) of semiconductor substrates 108. Device dies 102 may include logicdies such as Central Processing Unit (CPU) dies, Graphic Processing Unit(GPU) dies, mobile application dies, or the like.

Device dies 102 are molded in molding material 120, which surroundsdevice dies 102. Molding material 120 may be a molding compound, amolding underfill, a resin, or the like. The bottom surface 120A ofmolding material 120 may be leveled with the bottom ends of device dies102. The top surface 120B of molding material 120 may be level with orhigher than back surfaces 108A of semiconductor substrates 108. Inaccordance with some embodiments of the present disclosure, backsurfaces 108A of semiconductor substrates 108 are overlapped bydie-attach films 110, which adhere device dies 102 to the overlyingdielectric layer 118 and RDLs 116. Device dies 102 further include metalpillars 106 (which may include copper pillars) in contact with, andbonded to, RDLs 112.

Bottom package 100 may include Front-side RDLs 112 underlying devicedies 102 and back-side RDLs 116 overlying device dies 102. The term“front-side RDL” indicates that the respective RDLs are on the frontside of device dies 102, and the term “back-side RDL” indicates that therespective RDLs are on the back side of device dies 102. Front-side RDLs112 are formed in dielectric layers 114, and back-side RDLs 116 areformed in dielectric layers 118. RDLs 112 and 116 may be formed ofcopper, aluminum, nickel, alloys thereof, or multi-layers thereof. Inaccordance with some embodiments of the present disclosure, dielectriclayers 114 and 118 are formed of inorganic materials such as siliconoxide, silicon nitride, silicon oxynitride, or the like. In alternativeembodiments, dielectric layers 114 and 118 are formed of organicmaterials such as polymers, which may include polybenzoxazole (PBO),benzocyclobutene (BCB), polyimide, or the like.

Through-Vias 122 are formed to penetrate through molding material 120.In accordance with some embodiments of the present disclosure,through-vias 122 have top surfaces level with the top surface of moldingmaterial 120 and bottom surfaces level with the bottom surface ofmolding material 120. Through-Vias 122 electrically connect front-sideRDLs 112 and device dies 102A and 102B to back-side RDLs 116.Through-Vias 122 may also be in physical contact with some of front-sideRDLs 112 and back-side RDLs 116.

Electrical connectors 124, which are formed of a non-solder metallicmaterial(s), are formed at the bottom surface of bottom package 100. Inaccordance with some embodiments of the present disclosure, electricalconnectors 124 are metal pads. In alternative embodiments, electricalconnectors 124 include metal pillars such as copper pillars. Throughoutthe description, electrical connectors 124 are referred to as metal pads124, although they may have forms other than metal pads. Metal pads 124may comprise copper, aluminum, nickel, palladium, gold, or multi-layersthereof. Solder regions 126 are attached to the bottom surfaces of metalpads 124 and bond bottom package 100 to package component 300. In someexemplary embodiments, Under Bump Metallurgies (UBMs) 127 are formed atthe bottom surface of package component 100, with solder regions 126attached on. In alternative embodiments, no UBM is formed, and solderregions 126 are in contact with metal pads 124. Package component 300may include a Printed Circuit Board (PCB), a package, or another type ofpackage component.

The back-side RDLs 116 includes some metal pads 150. In accordance withsome embodiments, metal pads 150 are in the topmost RDL layer in packagecomponent 100. Polymer layer 152 is formed over RDLs 116 and dielectriclayers 118. Dielectric layer 152 may be formed of a polymer such as PBOor other organic or inorganic materials. Throughout the description,dielectric layer 152 is referred to as polymer layer 152 although it mayalso be formed of a dielectric material other than polymer. Inaccordance with some embodiments, tape 154 is over and attached todielectric layer 152. Tape 154 is used to provide protection andreinforcement to the underlying structure. Furthermore, laser marks 156may be formed in tape 154. Laser marks 156 are recesses/openings in tape154 and may be formed through laser. The top-view shape of laser marks156 may include letters, numbers, graphics, and/or the like. Hence,laser marks 156 may be used for identification purposes. In alternativeembodiments, tape 154 is not formed, and polymer layer 152 is the toplayer of package component 100.

Openings 158 are formed in polymer layer 152 and tape 154, and metalpads 150 are exposed to opening 158. Solder regions 206 have theirbottom portions filling openings 158, with solder regions 206 in contactwith metal pads 150.

Top package 200 is bonded to bottom package 100. In accordance with someembodiments of the present disclosure, top package 200 includes packagesubstrate 202 and device die 204, which is bonded to package substrate202. The bonding of device die 204 to package substrate 202 may beachieved through wire bonding, flip-chip bonding, or the like. Solderregions 206 bond top package 200 to bottom package 100. Furthermore,solder regions 206 are in contact with metal pads 208 at the bottomsurface of package component 200. Accordingly, solder regions 206 havetheir top surface in contact with metal pads 208 and bottom surfaces incontact with the top surfaces of metal pads 150.

FIG. 2 illustrates a schematic top view of portions of bottom package100, wherein openings 158 in polymer layer 152 (and tape 154, if any,FIG. 1) are illustrated. Openings 158 include elongated openings 158Aand non-elongated openings 158B. In FIGS. 2 through 9, circles are usedto schematically represent non-elongated openings 158B, and ovals areused to schematically represent elongated openings 158A. FIGS. 10A and10B illustrate top views of some exemplary elongated openings 158A. Ineach of FIGS. 10A and 10B, elongated opening 158A has length L1 measuredin lengthwise direction X and width W1 measured in widthwise directionY, which is perpendicular to lengthwise direction X. Length L1 isgreater than width W1. Elongated opening 158A includes longer axis 134in lengthwise direction X and shorter axis 136 in widthwise direction Y.Throughout the description, an elongated opening 158A is an openingwhose length-to-width ratio L1/W1 is greater than threshold ratio Ltthat is greater than 1. Threshold ratio Lt may be greater than about1.2, 1.6, or 2.0.

Elongated opening 158A may have various shapes including, but notlimited to, an oval shape, a rectangular shape, an elongated octagonalshape, or the like. For example, FIG. 10A illustrates an exemplaryelongated opening 158A, which includes two half circles 130 connected toopposite edges of rectangle 128. FIG. 10B illustrates an elongatedhexagon. It is appreciated that other elongated shapes other than whathave been discussed may also be used.

FIGS. 11A and 11B illustrate the top views of exemplary non-elongatedopenings 158B. Throughout the description, a non-elongated opening 158Bdoes not have length L2 and width W2 observably different from eachother. Alternatively, a non-elongated opening 158B has width W2 smallerthan length L2, with length-to-width ratio L2/W2 being smaller than thethreshold ratio Lt, which may be smaller than about 1.2 or about 1.1 inaccordance with some exemplary embodiments. For example, FIGS. 11A and11B illustrate non-elongated openings 158B, which have a circularbottom-view shape and a hexagonal bottom-view shape, respectively.Furthermore, in the same package 100, length-to-width ratio L1/W1 ofelongated openings 158A is greater than length-to-width ratio L2/W2 ofnon-elongated openings 158B. The top areas (which can be viewed fromFIGS. 10A, 10B, 11A, and 11B) of elongated openings 158A may be equal tothe top-view areas of non-elongated openings 158B, although they havedifferent shapes.

Although not shown in FIGS. 10A, 10B, 11A, and 11B, the top views ofsolder regions 206 (FIG. 1) are defined by, and may be the same as, thetop-view shapes of the respective underlying openings 158. Hence, solderregions 206 may also include elongated solder regions (in the top viewof package component 100) and non-elongated solder regions.

Referring back to FIG. 2, bottom package 100 includes four corners 138.The corner openings 158, which are closer to the respective corners 138than all other openings, are elongated openings 158A. Other openings 158that are farther away from the respective corners 138 than the corneropenings 158A are non-elongated openings 158B. In accordance with someembodiments of the present disclosure, there may be more than oneelongated opening 158A at each corner 138. For example, as shown in FIG.3, there are three elongated openings 158A at each corner 138.

FIGS. 3 and 4 illustrate the top views of bottom package 100 andopenings 158 in accordance with alternative embodiments. In the topview, bottom package 100 has neutral-stress point 140, which is thepoint substantially free from stresses from all lateral directions thatare parallel to the bottom surface of package 100. At neutral-stresspoint 140, the lateral stresses from opposite directions are cancelledout. The lateral stresses are the stresses parallel to the top andbottom surfaces of package component 100 in FIG. 1. In accordance withsome embodiments of the present disclosure, neutral-stress point 140 isat or close to the center of bottom package 100 (in the top view). Thedistance between each of openings 158 and neutral-stress point 140 isreferred to as a Distance to Neutral Point (DNP) of the respectiveopening 158, wherein the distance of an opening 158 is measured from apoint of the opening 158 that is closest to neutral-stress point 140.For example, DNPs DNP1 and DNP2 are illustrated as examples in FIG. 3.

As shown in FIGS. 3 and 4, a circle 142 is drawn with neutral-stresspoint 140 as the center, wherein circle 142 has radius r. In accordancewith the embodiments of the present disclosure, all openings 158 withDNPs equal to or smaller than radius r are designed as non-elongatedopenings 158B, and all openings 158 with DNPs greater than radius r aredesigned to be elongated openings 158A. As illustrated in FIG. 3, ifradius r is large, then the elongated openings include corner openings.In these embodiments, each row includes at least one (or more) openingthat is a non-elongated opening. In FIG. 4, radius r is reduced, andhence an entire edge-row or edge-column of openings 158 whose DNPs aregreater than radius r are elongated, while the openings 158 with theDNPs equal to or smaller than radius r are non-elongated. In theembodiments in FIGS. 3 and 4, the elongated openings 158A include corneropenings.

Radius r is determined based on the stresses suffered by solder regions206 and metal pads 150 (FIG. 1) and may be based on measurement resultsfrom packages and/or simulation results. In some embodiments, radius ris selected by ensuring the stresses suffered by all solder regions 206and metal pads 150 in circle 142 (FIGS. 3 and 4) to be lower than apre-determined threshold stress, while the stresses suffered by at leastsome solder regions 206 and metal pads 150 outside circle 142 are higherthan the threshold stress.

In accordance with some embodiments, openings 158 are distributed as anarray that is distributed throughout package 100, as shown in FIGS. 3through 5. In alternative embodiments, openings 158 are distributedclose to the peripheral regions of package 100, and are not in the innerregions. For example, FIGS. 2 and 3 schematically illustrate dashedrectangular regions 159. In these embodiments, openings 158 will beformed outside of rectangular regions 159, and will not be formed insiderectangular regions 159.

FIG. 5 illustrates the top view of some components in bottom package 100and openings 158 in accordance with yet alternative embodiments. Devicedie 102 is illustrated in the top view. Device die 102 includes corners146. Solder regions that are close to the corners 146 suffer from higherstresses than other nearby solder regions and hence are more prone tofailure. In accordance with some embodiments of the present disclosure,the inner openings 158 (which are not edge openings and not corneropening of package 100) that overlap (also refer to FIG. 1) device die102 and are also closest to corners 146 are elongated openings 158A,while other openings 158 (except the openings 158A discussed in theembodiments in FIGS. 2 through 4) are non-elongated openings 158B. Inaccordance with some embodiments of the present disclosure, the innerelongated openings 158A are surrounded by non-elongated openings 158B.

FIG. 6 illustrates some components in the top view of bottom package 100and openings 158 in accordance with yet alternative embodiments. Inthese embodiments, a plurality of device dies 102 is closely locatedfrom each other to form device die group 148. Device die group 148includes corners 146. In accordance with some embodiments of the presentdisclosure, there are four corner regions 147, each at a corner 146.There are four openings 158 in each of corner regions 147, wherein thefour openings 158 are closest to the respective corner 146. Each of thefour openings 158 in each corner region 147 includes one opening thatoverlaps die group 148 and three openings 158 that do not overlap diegroup 148. All four openings 158 in each corner region 147 are elongatedopenings 158A, while other openings 158 in the regions surrounding eachof corner regions 147 are non-elongated openings 158B.

There is also a plurality of edge regions 149 that overlaps the edges ofdie group 148. In each of edge regions 149, there are two rows ofopenings 158, with each of the two rows extending in the directionparallel to the respective edge of die group 148. One of the rows ofopenings 158 overlaps die group 148, and the other row of openings 158does not overlap die group. In accordance with some embodiments of thepresent disclosure, edge openings 158 in edge regions 149 arenon-elongated openings 158B. In alternative embodiments, edge openings158 in edge regions 149 are elongated openings 158A.

In accordance with some embodiments as shown in FIGS. 5 and 6, the inneropenings 158 that are overlapped by device die group 148, whose openings158 are also closest to corners 146, are elongated openings 158A, whileother openings 158 (except the openings 158A discussed in theembodiments in FIGS. 2 through 4) are non-elongated openings 158B. Inaccordance with some embodiments of the present disclosure, the innerelongated openings 158A are surrounded by non-elongated openings 158B.

In the above-discussed embodiments referring to FIG. 6, die group 148includes two dies. In alternative embodiments, die group 148 may includea single die or more than two dies. For example, as shown in FIG. 5,when die group 148 includes a single die, then the die corners 146 as inFIG. 6 will be the four corners of the single die.

FIGS. 7 and 8 illustrate the top views of bottom package 100 andopenings 158 in accordance with yet alternative embodiments. Theseembodiments are similar to the embodiments in FIGS. 5 and 6 except thatmore openings 158 that are close to the corners 146 of, and overlaps,device dies 102 or device die group 148 are elongated openings 158A,which are surrounded by non-elongated openings 158B. Furthermore, FIG. 7illustrates a single device die 102, with elongated openings 158A beingdistributed close to the corners 146 of device die 102. FIG. 8illustrates a device die group 148, with elongated openings 158A beingdistributed close to the corners 146 of device die group 148. Inaccordance with some embodiments in FIGS. 5 through 8, an innerelongated opening 158A may be fully, or partially, overlapped by theoverlying device die 102 or device die group 148.

FIG. 9 illustrates the design of openings 158 in accordance with yetalternative embodiments. In these embodiments, four corner regions 160of bottom package 100 are defined, each extending from one of corners138 inwardly. The four corner regions 160 may have rectangular shapesand may have sizes the same as each other. The openings 158 insidecorner regions 160 are designed as elongated opening 158A. The openings158 outside corner regions 160 may be designed as non-elongated openings158B or may be designed as elongated opening 158A.

In some embodiments as in FIG. 9, circle 142 is also drawn according tosimulation or experiment results. The radius of circle 142 may be small,and hence some of openings 158 that are outside of corner regions 160are also outside of circle 142. Accordingly, as shown in FIG. 6, some ofopenings 158 (marked as 158″) that are outside of the circle 142 arealso elongated openings 158A, while the openings 158 that are outside ofcorner regions 160, but inside circle 142, are non-elongated openings158B.

FIGS. 12A and 12B illustrate the top views of some exemplary elongatedopenings 158A and the respective metal pad 150 (also refer to FIG. 1)that are underlying and in contact with the elongated openings 158A. Inaccordance with some embodiments, metal pad 150 has a non-elongatedshape such as a circle (as shown in FIG. 12A), a square, a hexagon, anoctagon, or the like. In alternative embodiments, as shown in FIG. 12B,metal pad 150 may also have an elongated shape that fits the shape ofthe respective elongated opening 158A. Furthermore, the long axis ofmetal pad 150 may be parallel to, and may overlap, the long axis of therespective elongated opening 158A. By designing metal pad 150 to beelongated, the reliability of the respective package may be furtherimproved.

In the above-discussed embodiments, openings 158 include elongatedopenings 158A and non-elongated openings 158B. As shown in FIG. 1,openings 158 (including elongated openings 158A and non-elongatedopenings 158B) are bonded to metal pads 208 through solder regions 206.To fit the shapes of elongated openings 158A and non-elongated openings158B, metal pads 208 in package component 200 may also be designed tohave elongated metal pads 208A overlapping and bonded to elongatedopenings 158A and elongated metal pads 208B overlapping and bonded tonon-elongated openings 158B. An exemplary bottom view of metal pads 208Aand 208B are schematically illustrated in FIG. 13. In accordance withsome embodiments of the present disclosure, each of the elongatedopenings 158A in FIGS. 2 through 9 corresponded to, and are overlappedby and bonded to, the respective overlying elongated metal pads 208A,and each of the non-elongated openings 158B in FIGS. 2 through 9corresponded to, and are overlapped by, the respective overlyingnon-elongated metal pads 208B. The bottom shapes of elongated metal pads208A and non-elongated metal pads 208B and their respective positionsare similar to the shape of the corresponding elongated openings 158Aand non-elongated openings 158B in FIGS. 2 through 9, and hence are notillustrated.

Throughout the embodiments of the present disclosure, as shown in FIGS.2 through 9, elongated openings 158A are centripetal. In accordance withsome embodiments, the longer axis 134 (FIGS. 10A and 10B) of elongatedcentripetal openings 158A extend toward the neutral-stress point 140(FIGS. 2 through 4), which may be at, or at least close to, the centerof bottom package 100 (in the top view). Alternatively stated, thelonger axis 134 (FIGS. 10A and 10B) of the centripetal elongatedopenings 158A pass through the center of bottom package 100, or atleast, the longer axis 134 of the centripetal elongated openings 158Aare closer to the center of bottom package 100 than the respectiveshorter axis 136.

The embodiments of the present disclosure have some advantageousfeatures. By designing centripetal elongated openings, the solderregions in the centripetal elongated openings can endure higher stresseswithout failure than the solder regions in non-elongated openings. Thelocations of the centripetal elongated openings are selected accordingto the stresses suffered by the solder regions. Simulation resultsindicated that when the corner openings of the bottom package 100 arecentripetal elongated openings, the respective package fails after 769thermal cycles in the reliability test. In comparison, when the corneropenings of the bottom package 100 are non-elongated openings, therespective package fails after 604 thermal cycles. When the corneropenings of the bottom package 100 are elongated openings with thewidthwise directions extending toward the neutral-stress point, therespective bottom package fails after 574 thermal cycles. These resultsindicate that a package with centripetal elongated openings haveimproved reliability and can endure more thermal circles before theyfail.

FIGS. 14 through 26 illustrate cross-sectional views and top views ofintermediate stages in the formation of packages in accordance withalternative embodiments of the present disclosure. Unless specifiedotherwise, the materials and the formation methods of the components inthese embodiments are essentially the same as the like components, whichare denoted by like reference numerals in the embodiments shown in FIGS.1 through 13. The details regarding the formation process and thematerials of the components shown in FIGS. 14 through 26 may thus befound in the discussion of the embodiments shown in FIGS. 1 through 13.

Referring to FIG. 14, bottom package 100 is formed. The majority ofbottom package 100 is similar to what is shown in FIG. 1, and thedetails of these parts are not described. Backside RDLs 116 are formedon the back side of device die 102. In accordance with some embodimentsof the present disclosure, there is one device die 102 in bottom package100, as illustrated in FIG. 14. In accordance with alternativeembodiments of the present disclosure, there are two or more device diesin bottom package 100, similar to what is shown in FIG. 1. Furthermore,although one layer of RDLs 116 is illustrated, there may be two or morelayers of RDLs on the backside of device die 102, depending on therouting requirement of the respective package. RDLs 116 are formed indielectric layer(s) 118. In accordance with some embodiments of thepresent disclosure, dielectric layer 118 is formed of an organicmaterial such as a polymer, which may include PBO, polyimide, BCB, orthe like. In accordance with alternative embodiments of the presentdisclosure, dielectric layer 118 is formed of an inorganic dielectricmaterial such as silicon oxide, silicon nitride, silicon oxynitride,multi-layers thereof, or combinations thereof.

Backside RDLs 116 include conductive lines and pads 162 (collectivelyreferred to as lines/pads 162 hereinafter), and vias 164 electricallyconnecting conductive lines/pads 162 to through-vias 122. In accordancewith some embodiments, all top surfaces of conductive lines/pads 162 areplanar, and are coplanar with the top surface of dielectric layer 118.In accordance with alternative embodiments, each of bonding pads 162 hasa recessed center portion (not shown), and edges portions higher thanthe recessed center portion. Conductive lines/pads 162 may be formed ofaluminum, nickel, titanium, aluminum copper, or the like. Throughout thedescription, bonding pads 162 are alternatively referred to as bondingfeatures.

As also shown in FIG. 14, dielectric layer 131 is formed. The bottomsurface of dielectric layer 131 is in contact with the top surfaces ofdielectric layer 118 and conductive lines/pads 162. In accordance withsome exemplary embodiments of the present disclosure, dielectric layer131 is formed of a polymer, and hence is referred to as polymer layer131 throughout the description. It is appreciated that dielectric layer131 may also be formed of a non-polymer material. The exemplarycandidate materials for forming polymer layer 131 include, but are notlimited to, PBO, BCB, polyimide, and the like. In accordance with someembodiments, both dielectric layers 118 and 131 are formed of PBO.

Referring to FIG. 15, an etching step is performed to remove dielectriclayer 131, wherein the etching is illustrated using arrows. Inaccordance with some embodiments of the present disclosure, the etchingis isotropic, and may be performed using dry etching or wet etching. Inthe embodiments wherein dielectric layers 118 and 131 are formed of PBO,the etching may be performed in a dry etching process, with the gasesused for the dry etching including nitrogen (N₂), argon (Ar), and oxygen(O₂). In accordance with alternative embodiments, a CMP is performed toremove dielectric layer 131.

FIG. 16 illustrates the bottom package 100 after the etching. In theetching process, dielectric layer 131 (FIG. 15) is removed, andconductive lines/pads 162 are exposed. Furthermore, the etching processis controlled to have an over-etching, so that the top portion ofdielectric layer 118 is also etched, and the bottom portion ofdielectric layer 118 is not etched. Accordingly, dielectric layer 118 isrecessed. Recessing depth D1 may be in the range between about ⅓ andabout ½ of thickness T1 of conductive lines/pads 162. The recessing ofdielectric layer 118 ensures the exposure of conductive lines/pads 162.Furthermore, with the recessing of dielectric layer 118, the sidewallsof bonding pads 162 are exposed, and hence the reliability of thesubsequent bonding process is improved.

Next, referring to FIG. 17, package 100 is bonded with package 200. Inaccordance with some embodiments of the present disclosure, the bondingis performed through solder regions 206, which join bonding pads 162 tometal pads 208 in the overlying package 200. In some embodiments,package 200 includes device dies 204, which may be memory dies such asStatic Random Access Memory (SRAM) dies, Dynamic Random Access Memory(DRAM) dies, or the like. Memory dies 204 may also be bonded to packagesubstrate 202 in some exemplary embodiments.

Due to the recessing of dielectric layer 118, solder regions 206 arealso in contact with the sidewalls of bonding pads 162. The interfacebetween solder region 206 and the sidewall of some of bonding pads 162may form full rings. Advantageously, with solder regions 206 contactingthe sidewalls of bonding pads 162, the contact areas between bondingpads 162 and solder regions 206 are increased, and hence the contactresistance is reduced. Furthermore, the bonding strength is improved.

FIG. 18A illustrates the dispensing of underfill 166 between packages100 and 200. Underfill 166 encircles solder regions 206. Underfill 166is dispensed in a liquid state, and is then cured, for example, in athermal process. In accordance with some embodiments of the presentdisclosure, the bottom surface of underfill 166 is in contact with thetop surface of dielectric layer 118. Furthermore, the bottom surface ofunderfill 166 is lower than the top surfaces of bonding pads 162 in theembodiments in which dielectric layer 118 is recessed. The top surfaceof underfill 166 is in contact with the bottom surface of package 200.The entirety of underfill 166 may be formed of a homogenous material.

FIG. 18B illustrates a schematic top view of some portions of bottompackage 100, wherein bonding pads 162 are illustrated. Bonding pads 162include elongated bonding pads 162A and non-elongated bonding pads 162B.In the following figures, circles are used to schematically representnon-elongated bonding pads 162B, and ovals are used to schematicallyrepresent elongated bonding pads 162A.

FIGS. 19A and 19B illustrate the top views of some exemplary elongatedbonding pads 162A. In each of FIGS. 19A and 19B, elongated bonding pad162A has length L1′ measured in lengthwise direction X and width W1′measured in widthwise direction Y, which is perpendicular to lengthwisedirection X. Length L1′ is greater than width W1′. Elongated bonding pad162A includes longer axis 134′ in lengthwise direction X and shorteraxis 136′ in widthwise direction Y. Throughout the description, anelongated bonding pad 162A is a pad whose length-to-width ratio L1′/W1′is greater than threshold ratio Lt, which may be greater than about 1.2,1.6, or 2.0.

Elongated bonding pad 162A may have various shapes including, but notlimited to, an oval shape, a rectangular shape, an elongated octagonalshape, or the like. For example, FIG. 19A illustrates an exemplaryelongated bonding pad 162A, which includes two half circles 130′connected to opposite edges of rectangle 128′. FIG. 19B illustrates anexemplary elongated bonding pad 162A with a hexagon shape. It isappreciated that other elongated shapes other than what have beendiscussed may also be used.

FIGS. 20A and 20B illustrate the top views of exemplary non-elongatedbonding pads 162B. Throughout the description, a non-elongated bondingpad 162B does not have length L2′ and width W2′ significantly differentfrom each other. Alternatively, a non-elongated bonding pad 162B haswidth W2′ smaller than length L2′, with length-to-width ratio L2′/W2′being smaller than the threshold ratio Lt, which may be smaller thanabout 1.2 or about 1.1 in accordance with some exemplary embodiments.For example, FIGS. 20A and 20B illustrate non-elongated bonding pads162B, which have a circular bottom-view shape and a hexagonalbottom-view shape, respectively. In the same package 100,length-to-width ratio L1′/W1′ of elongated bonding pads 162A is greaterthan length-to-width ratio L2/W2 of non-elongated bonding pads 162B.

Although not shown in FIGS. 19A, 19B, 20A, and 20B, the top views ofsolder regions 206 (FIG. 20) are defined by bonding pads 162. Hence,solder regions 206 may also include elongated solder regions (in the topview of package component 100) and non-elongated solder regions.

Referring back to FIG. 18B, bottom package 100 includes four corners138. The corner bonding pads 162, which are closer to the respectivecorners 138 than all other bonding pads, are elongated bonding pads162A. Other bonding pads 162 that are farther away from the respectivecorners 138 than the corner bonding pads 162A may be non-elongatedbonding pads 162B. In accordance with some embodiments of the presentdisclosure, there may be more than one elongated bonding pad 162A ateach corner 138. For example, as shown in FIG. 18B, there are threeelongated bonding pads 162A at each corner 138. In accordance withalternative embodiments, there is a single elongated bonding pad 162A ateach corner 138, while other bonding pads are non-elongated.

Whether a bonding pad 162 should be designed as elongated ornon-elongated is related to its distance from the neutral-stress pointof package 100 in accordance with some embodiments of the presentdisclosure. As shown in FIG. 18B, in the top view, bottom package 100has neutral-stress point 140, which is the point substantially free fromthe stresses applied from any lateral direction that is parallel to thebottom surface of package 100. At neutral-stress point 140, the lateralstresses from opposite directions are cancelled out. The lateralstresses are the stresses parallel to the top and bottom surfaces ofpackage component 100 in FIG. 18A. In accordance with some embodimentsof the present disclosure, neutral-stress point 140 is at or close tothe center of bottom package 100 (in the top view). The distance betweeneach of bonding pads 162 and neutral-stress point 140 is referred to asa DNP of the respective bonding pad 162, wherein the distance of abonding pad 162 is measured from a point of the bonding pad 162 that isclosest to neutral-stress point 140. For example, DNPs DNP1′ and DNP2′are illustrated as examples in FIG. 18B.

As shown in FIG. 18B, a circle 142 (such as 142A) may be drawn withneutral-stress point 140 as the center, wherein circle 142 has radius r.In accordance with the embodiments of the present disclosure, allbonding pads 162 with DNPs equal to or smaller than radius r aredesigned as non-elongated bonding pads 162B, and all bonding pads 162with DNPs greater than radius r are designed to be elongated bondingpads 162A. If radius r is large, then the elongated conductive padsinclude four corner pads but not other bonding pads. When radius r ischosen to be smaller (as shown by a smaller ring 142B), more bondingpads 162 are designed to be elongated.

The selection of the appropriate radius r is related to the stress levelof solder regions 206, and with the appropriate radius r selected, nosolder regions 206 should have cracking caused by the stress. Radius rmay also be based on the measurement results from packages and/orsimulation results. In accordance with some embodiments, radius r isselected by ensuring the stresses suffered by all solder regions 206 andbonding pads 162 in circle 142 to be lower than a pre-determinedthreshold stress, while the stresses suffered by at least some solderregions 206 and bonding pads 162 outside circle 142 are higher than thethreshold stress.

In accordance with some embodiments of the present disclosure, bondingpads 162 are distributed close to the peripheral regions of package 100,and not in the inner regions, as shown in FIG. 18B. In accordance withalternative embodiments, bonding pads 162 are distributed as an arraythat expands throughout package 100, as shown in FIG. 18C.

As shown in FIGS. 18B and 18C, elongated bonding pads 162A arecentripetal, wherein the lengthwise directions of elongated bonding pads162A extend toward center (neutral-stress point) 140 of package 100. Inaccordance with some embodiments, the lengthwise directions (Xdirections in FIGS. 19A and 19B) of elongated bonding pads 162A may passthrough center 140 of package 100.

It is realized that the amount of solder needed by solder regions 206 isrelated to the top-view areas of bonding pads 162. The heights of solderregions 206 are related to the areas of bonding pads 162, and thegreater the areas of bonding pads 162, the lower the heights of solderregions 206 will be. Since the volumes of solder regions 206 are likelyto be the same as each other, the solder regions 206 formed on largebonding pads 162 are more likely to have lower heights, and are morelikely to have inferior contact with bonding pads 162 and/or metal pads208. To avoid this problem, elongated bonding pads 162A andnon-elongated bonding pads 162B are designed to have the same area (thetop-view area in FIG. 18A). Accordingly, length L1′ of elongated bondingpads 162A (FIG. 19A and 19B) is greater than length L2′ of non-elongatedbonding pads 162B (FIG. 20A or 20B), and width W2′ of elongated bondingpads 162A (FIG. 19A or 19B) is smaller than width W2′ of non-elongatedbonding pads 162B (FIG. 20A or 20B).

FIGS. 21 through 25 illustrate the cross-sectional views of intermediatestages in the formation of a package in accordance with alternativeembodiments. These embodiments are similar to the embodiments in FIGS.14 through 18C, except that no backside RDLs are formed in theseembodiments, and solder regions are bonded directly to through-vias.

Referring to FIG. 21, bottom package 100 is formed. In accordance withthese embodiments of the present disclosure, through-vias 122,die-attach film 110, and molding material 120 have top surfaces coplanarwith each other. Dielectric layer 131 is formed to cover through-vias122, die-attach film 110, and molding material 120. Furthermore,dielectric layer 131 may be in physical contact with the top surfaces ofthrough-vias 122, die-attach film 110, and molding material 120. Thematerial of dielectric layer 131 may be selected from the same candidatematerials for forming dielectric layer 131 in FIG. 14. In accordancewith some embodiments, dielectric layer 131 is formed of PBO.

Referring to FIG. 22, an etching step is performed to remove dielectriclayer 131, wherein the etching is illustrated using arrows. Inaccordance with some embodiments of the present disclosure, the etchingis isotropic, and may be performed using dry etching or wet etching. Inaccordance with alternative embodiments, a CMP is performed to removedielectric layer 131. In the embodiments in which dielectric layer 131is formed of PBO, the etching may include a dry etching process, withthe gases used for the dry etching including nitrogen (N₂), argon (Ar),and oxygen (O₂).

FIG. 23 illustrates the bottom package 100 after the etching. In theetching process, dielectric layer 131 (FIG. 15) is removed, andthrough-vias 122 are exposed. Throughout the description, through-vias122 are alternatively referred to as bonding features. Furthermore, theetching process is controlled to have an over-etching, so that the topportion of molding material 120 is also recessed. The recessing ofmolding material 120 ensures the exposure of the top portions ofsidewalls of through-vias 122, and hence the reliability of thesubsequently performed bonding process is improved. As a result of therecessing of molding material 120, the sidewalls of die-attach film 110may also be exposed.

Next, referring to FIG. 24, package 100 is bonded with package 200. Inaccordance with some embodiments, the bonding is performed throughsolder regions 206, which join through-vias 122 to metal pads 208 in theoverlying package 200. Due to the recessing of molding material 120 andthe exposure of the sidewalls of through-vias 122, solder regions 206are also in contact with the sidewalls of through-vias 122, resulting inthe increase in the contact area between through-vias 122 and solderregions 206. Accordingly, the contact resistance is reduced.Furthermore, the bonding strength is improved.

FIG. 25 illustrates the dispensing of underfill 166, which encirclessolder regions 206. In accordance with some embodiments of the presentdisclosure, the bottom surface of underfill 166 is in contact with thetop surface of die-attach film 110 and molding material 120. Underfill166 is further in contact with the sidewalls of die-attach film 110.Furthermore, the bottom surface of underfill 166 is lower than the topsurfaces of through-vias 122 in the embodiments in which moldingmaterial 120 is recessed. The top surface of underfill 166 is in contactwith the bottom surface of package 200. The entirety of underfill 166may be formed of a homogenous material.

FIG. 26 illustrates a schematic top view of some portions of bottompackage 100, wherein through-vias 122 are illustrated. Similar tobonding pads 162, through-vias 122 include elongated through-vias 122Aand non-elongated through-vias 122B, as viewed in the top views orbottom views of package 100. Elongated through-vias 122A are alsocentripetal, with the respective lengthwise directions pointing toneutral stress point 140. Again, circles are used to schematicallyrepresent non-elongated through-vias 122B, and ovals are used toschematically represent elongated through-vias 122A. The distinctionbetween elongated through-vias 122A and non-elongated through-vias 122Bis similar to the distinction of elongated bonding pads 162A andnon-elongated bonding pads 162B, as shown in FIGS. 19A, 19B, 20A, and20B. For example, in the same package 100, length-to-width ratio L1′/W1′of elongated through-vias 122A is greater than length-to-width ratioL2′/W2′ of non-elongated through-vias 122B.

The placement of elongated through-vias 122A and non-elongatedthrough-vias 122B may also be similar to the placement of elongatedbonding pads 162A and non-elongated bonding pads 162B. For example, asshown in FIG. 26, bottom package 100 includes four corners 138. Thecorner through-vias 122, which are closer to the respective corners 138than all other through-vias 122, are elongated through-vias 122A. Otherthrough-vias 122 that are farther away from the respective corners 138than the corner through-vias 122A may be non-elongated through-vias122B. In accordance with some embodiments of the present disclosure,there may be more than one elongated through-via 122A at each corner138. For example, as shown in FIG. 26, there are three elongatedthrough-vias 122A at each corner 138. In accordance with alternativeembodiments, there is a single elongated through-via 122A at each corner138, while other through-vias are non-elongated. In yet alternativeembodiments, whether a through-via is designed as elongate or not isbased on its DNP, similar to the design of bonding pads 162 in FIG. 18B,and more or fewer elongated through-vias 122A may be designed dependingon the stress level in package 100.

FIGS. 27A and 27B illustrate the bottom views of metal pads 208 (FIG.25) in package 200 in accordance with some embodiments. Metal pads 208may include elongated metal pads 208A and non-elongated metal pads 208B.Elongated metal pads 208A are centripetal, with the respectivelengthwise directions pointing to neutral-stress point 240 of package200. Referring to FIG. 27A, in accordance with some embodiments of thepresent disclosure, each of metal pads 208A is joined to the same solderregion 206 as the respective underlying elongated bonding pad 162A (orelongated through-vias 122A, referring to FIG. 18B, 18C, or 25), andvice versa. Each of non-elongated metal pads 208B may be joined to thesame solder region 206 as the respective underlying non-elongatedbonding pad 162B (or non-elongated through-vias 122B, referring to FIG.18B, 18C, or 25), and vice versa.

Referring to FIG. 27B, in accordance with alternative embodiments of thepresent disclosure, all metal pads 208 are non-elongated, and may havethe same bottom-view shape and size. As a result, elongated bonding pads162A (or elongated through-vias 122A) and non-elongated bonding pads162B (or non-elongated through-vias 122B) are all connect to (throughsolder regions 206) non-elongated metal pads 208B. In FIGS. 27A and 27B,there are metal pads 208 disposed inside rectangles 259. In accordancewith alternative embodiments, there is no metal pad 208 disposed insiderectangles 259.

The embodiments of the present disclosure have some advantageousfeatures. By designing centripetal elongated bonding pads orthrough-vias, the solder regions can endure higher stresses withoutfailure than the solder regions joined to non-elongated bonding pads orthrough-vias. The locations of the centripetal elongated bonding pads orthrough-vias are selected according to the stresses suffered by thesolder regions. Simulation results indicate that a package withcentripetal bonding pads or through-vias have improved reliability andcan endure more thermal circles before they fail.

In accordance with some embodiments of the present disclosure, a packageincludes a corner, a device die, a molding material molding the devicedie therein, and a plurality of bonding features. The plurality ofbonding features includes a corner bonding feature at the corner,wherein the corner bonding feature is elongated. The plurality ofbonding features further includes an additional bonding feature, whichis non-elongated.

In accordance with alternative embodiments of the present disclosure, apackage includes a device die having a front side and a backside, amolding material molding the device die therein, a dielectric layer onthe backside of the device die, and a plurality of bonding pads. Each ofthe plurality of bonding pads has a portion in the dielectric layer. Theplurality of bonding pads includes a corner bonding pad at the corner,wherein the corner bonding pad is elongated in a top view of the firstpackage, and is centripetal, with a lengthwise direction extendingtoward a center of the first package. The plurality of bonding padsfurther includes an additional bonding pad farther away from the cornerthan the corner bonding pad. The additional bonding pad is non-elongatedin the top view of the first package. A first solder region is bonded tothe corner bonding pad. A second solder region is bonded to theadditional bonding pad.

In accordance with alternative embodiments of the present disclosure, amethod includes etching a first dielectric layer of a package to exposesurfaces of a first bonding feature and a second bonding feature in thefirst package. The first bonding feature is elongated, and the secondbonding feature is non-elongated. The package includes a device die, amolding material molding the device die therein, and a plurality ofthrough-vias penetrating through the molding material. The methodfurther includes forming a first solder region and a second solderregion contacting the surfaces of the first bonding feature and thesecond bonding feature.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a first package comprising:a corner; a device die; a molding material molding the device dietherein; and a plurality of bonding features comprising: a first bondingfeature at the corner, wherein the first bonding feature is elongated ina top view of the first package; and a second bonding feature, whereinthe second bonding feature is non-elongated, and wherein the firstbonding feature and the second bonding feature have a same top-viewarea.
 2. The package of claim 1, wherein the first bonding feature andthe second bonding feature comprise a first bonding pad and a secondbonding pad, respectively.
 3. The package of claim 1, wherein the firstbonding feature and the second bonding feature comprise a firstthrough-via and a second through-via, respectively, with the firstthrough-via and the second through-via penetrating through the moldingmaterial.
 4. The package of claim 1 further comprising: a secondpackage; and a first solder region bonding the first bonding feature tothe second package; and a second solder region bonding the secondbonding feature to the second package.
 5. The package of claim 4,wherein the first solder region and the second solder region are incontact with a first sidewall of the first bonding feature and a secondsidewall of the second bonding feature, respectively.
 6. The package ofclaim 5, wherein an interface between the first solder region and thefirst sidewall of the first bonding feature form a ring.
 7. The packageof claim 4, wherein the second package comprises a metal pad in contactwith the first solder region, and wherein the metal pad is elongated intop view of the second package, and has a length and a width smallerthan the length.
 8. The package of claim 1 further comprising: adie-attach film on a back surface of the device die, wherein thedie-attach film comprises a first portion protruding outside of themolding material; and an underfill in contact with the molding materialand the die-attach film.
 9. The package of claim 8, wherein thedie-attach film further comprises a second portion in the moldingmaterial.
 10. A package comprising: a first package comprising: acorner; a device die; a molding material molding the device die therein;and a plurality of bonding features comprising: a first bonding featureat the corner, wherein the first bonding feature is elongated in a topview of the first package; and a second bonding feature, wherein thesecond bonding feature is non-elongated, and wherein the length of thefirst bonding feature is greater than the length of the second bondingfeature, and the width of the first bonding feature is smaller than thewidth of the second bonding feature.
 11. The package of claim 10,wherein the first bonding feature and the second bonding featurecomprise a first bonding pad and a second bonding pad, respectively. 12.The package of claim 10, wherein the first bonding feature and thesecond bonding feature comprise a first through-via and a secondthrough-via, respectively, with the first through-via and the secondthrough-via penetrating through the molding material.
 13. The package ofclaim 10, wherein the plurality of bonding features comprises aplurality of non-elongated bonding pads encircling a region free frombonding pads, and wherein the region has a top-view area greater than anarea occupied by an array comprising at least four bonding pads.
 14. Thepackage of claim 10 further comprising a dielectric layer on a backsideof the device die, wherein the first bonding feature comprises a firstportion in the dielectric layer, and a second portion protruding beyondthe dielectric layer.
 15. The package of claim 10 further comprising: asecond package; and a first solder region and a second solder regionrespectively bonding the first bonding feature and the second bondingfeature to the second package, wherein the second package comprises ametal pad in contact with the first solder region, and wherein the metalpad is elongated.
 16. The package of claim 15, wherein the first solderregion and the second solder region are in contact with a sidewall ofthe first bonding feature and a sidewall of the second bonding feature,respectively.
 17. A package comprising: a first package comprising: acorner; a device die; a molding material molding the device die therein;a plurality of bonding features comprising: a first bonding feature atthe corner, wherein the first bonding feature is elongated in a top viewof the first package; and a second bonding feature, wherein the secondbonding feature is non-elongated; a first solder region on the firstbonding feature; and a second solder region on the second bondingfeature, wherein a volume of the first solder region is the same as avolume of the second solder region.
 18. The package of claim 17, whereinthe first bonding feature is closest to the corner of the first packagethan all other bonding features in the first package.
 19. The package ofclaim 17, wherein the first bonding feature and the second bondingfeature comprise a first bonding pad and a second bonding pad,respectively.
 20. The package of claim 17, wherein the first bondingfeature and the second bonding feature comprise a first through-via anda second through-via, respectively, with the first through-via and thesecond through-via penetrating through the molding material.